The present invention relates to a method of improving characteristics of a cation-exchange membrane. More particularly, it relates to a method of improving characteristics in an electrolysis of an electrolyte by a cation-exchange membrane in an aqueous solution having hydroxyl group ions which comprises swelling a homogeneous cation-exchange membrane with a water-miscible organic solvent to reduce the permeability of hydroxyl group ions and removing the water-miscible organic solvent and treating it with hydrochloric acid.
Heretofore, the cation-exchange membranes have been used in a concentration of an alkali hydroxide by an electrodialysis in an electrolysis of an aqueous solution of sodium sulfate and in a demineralization of brine or sea water for producing fresh water and they also used as a diaphragm for an electrolysis of sodium chloride.
The following characteristics are required for the cation-exchange membrane.
(1) high cation transport number of the membrane PA1 (2) low electric resistance of the membrane PA1 (3) high chemical stability PA1 (4) high mechanical strength PA1 R.sub.2, R.sub.3 =H, or R.sub.1 ; PA1 M=alkali metal; PA1 A'=--SO.sub.2 X', --COX'; PA1 X'=F or --OY; PA1 Y=H, M, NH.sub.4, or R.sub.1 ; PA1 X.sub.1 =F, --CH.sub.3 or --CF.sub.2 --O--CH.sub.3 PA1 h=2 to 12; PA1 h.sub.1 =0 or 1; h.sub.2 =0 or 1 to 5; h.sub.3 =0 or 1; PA1 h.sub.4 =1 to 5.
Cation-exchange membranes of fluorinated polymer having sulfonic acid groups such as Nafion (E. I. Dupont) have been well-known as cation-exchange membranes which are practically used.
The cation-exchange membranes have satisfactory characteristics of (3) and (4) but do not have satisfactory characteristics of (1) and (2).
The inventors have studied the development of the cation-exchange membranes having satisfactory characteristics of (1) and (2), and have found the cation-exchange membranes of fluorinated polymer having cation-exchangeable groups which have excellent characteristics.
Many methods of improving characteristics of cation-exchange membranes in various uses by reducing permeability of hydroxyl group ions have been proposed. (Japanese Unexamined Patent Publication Nos. 44360/1973, 66488/1975, 105581/1975, 108182/1975 and 120492/1975, etc.)
In these methods, the chemical structure of a part or whole of the membrane are modified or another polymer is combined or special ion-exchangeable groups are formed. They are substantially different from the method of the invention.
In these methods, it may be difficult to recover a membrane whose functional characteristic is deteriorated by the application. In accordance with the method of the present invention, the cation-exchange membrane can be easily recovered to the initial characteristic or higher.
A treatment of an ion-exchange membrane with an organic solvent is disclosed in Japanese Unexamined Patent Publication No. 158590/1975.
The ion-exchange membranes described in the specification are heterogeneous type ion-exchange membranes having a hydrocarbon type polymer which is electrically inactive bonding substrate or a net structure polymer. Accordingly, the membranes are quite different from the cation-exchange membrane which can not impart the effect of the present invention and the treatment is quite different. Accordingly, the concept and effect of the present invention are clearly different from those of Japanese Unexamined Patent Publication No. 158590/1975.
When the cation-exchange membranes of fluorinated polymer are treated, the effects of the present invention will be expected.